Method of fabricating an emitter region of a solar cell

ABSTRACT

Methods of fabricating emitter regions of solar cells are described. Methods of forming layers on substrates of solar cells, and the resulting solar cells, are also described.

The invention described herein was made with Governmental support undercontract number DE-FC36-07G017043 awarded by the United StatesDepartment of Energy. The Government may have certain rights in theinvention.

TECHNICAL FIELD

Embodiments of the present invention are in the field of renewableenergy and, in particular, methods of fabricating emitter regions ofsolar cells.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of the substrate creates electron and hole pairs in the bulkof the substrate, which migrate to p-doped and n-doped regions in thesubstrate, thereby generating a voltage differential between the dopedregions. The doped regions are connected to metal contacts on the solarcell to direct an electrical current from the cell to an externalcircuit coupled thereto.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the solar cell's capability to generate power.Accordingly, techniques for increasing the efficiency of solar cells aregenerally desirable. Embodiments of the present invention allow forincreased solar cell efficiency by providing novel processes forfabricating solar cell structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a flowchart representing operations in a method offabricating an emitter region of a solar cell, in accordance with anembodiment of the present invention.

FIG. 2A illustrates a cross-sectional view of a stage in the fabricationof a solar cell including an emitter region, in accordance with anembodiment of the present invention.

FIG. 2B illustrates a cross-sectional view of a stage in the fabricationof a solar cell including an emitter region, corresponding to operation102 of the flowchart of FIG. 1, in accordance with an embodiment of thepresent invention.

FIG. 2C illustrates a cross-sectional view of a stage in the fabricationof a solar cell including an emitter region, corresponding to operation104 of the flowchart of FIG. 1, in accordance with an embodiment of thepresent invention.

FIG. 2D illustrates a cross-sectional view of a stage in the fabricationof a solar cell including an emitter region, corresponding to operation106 of the flowchart of FIG. 1, in accordance with an embodiment of thepresent invention.

FIG. 2E illustrates a cross-sectional view of a stage in the fabricationof a solar cell including an emitter region, corresponding to operation108 of the flowchart of FIG. 1, in accordance with an embodiment of thepresent invention.

FIG. 3 illustrates a flowchart representing operations in a method offorming layers on a substrate of a solar cell, in accordance with anembodiment of the present invention.

FIG. 4A illustrates a cross-sectional view of a stage in the fabricationof solar cells, corresponding to operation 302 of the flowchart of FIG.3, in accordance with an embodiment of the present invention.

FIG. 4B illustrates a magnified view of a portion of FIG. 4A, inaccordance with an embodiment of the present invention.

FIG. 4C illustrates a cross-sectional view of a stage in the fabricationof solar cells, corresponding to operations 304 and 306 of the flowchartof FIG. 3, in accordance with an embodiment of the present invention.

FIG. 5 illustrates both a cross-sectional view and a top-down view of asubstrate of a solar cell, the substrate having layers formed thereon,in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Methods of fabricating emitter regions of solar cells are describedherein. In the following description, numerous specific details are setforth, such as specific process flow operations, in order to provide athorough understanding of embodiments of the present invention. It willbe apparent to one skilled in the art that embodiments of the presentinvention may be practiced without these specific details. In otherinstances, well-known fabrication techniques, such as lithographic andetch techniques, are not described in detail in order to notunnecessarily obscure embodiments of the present invention. Furthermore,it is to be understood that the various embodiments shown in the figuresare illustrative representations and are not necessarily drawn to scale.

Disclosed herein are methods of fabricating emitter regions of solarcells. In one embodiment, a method of fabricating an emitter region of asolar cell includes forming, in a furnace, a tunnel oxide layer on asurface of a substrate. Without removing the substrate from the furnace,an amorphous layer is formed on the tunnel oxide layer. The amorphouslayer is doped to provide a first region having N-type dopants and asecond region having P-type dopants. Subsequently, the amorphous layeris heated to provide a polycrystalline layer having an N-type-dopedregion and a P-type-doped region. In one embodiment, a method of forminglayers on a substrate of a solar cell includes loading, into a furnace,a wafer carrier with a plurality of wafers, the wafer carrier having oneor more wafer receiving slots loaded with two wafers positionedback-to-back. In the furnace, a tunnel oxide layer is formed on allsurfaces of each of the plurality of wafers. Without removing thesubstrate from the furnace, an amorphous layer is formed on the tunneloxide layer, the amorphous layer formed on all portions of the tunneloxide layer except on the portions in contact between wafers positionedback-to-back.

Also disclosed herein are solar cells. In such embodiments, a solar cellincludes a substrate or wafer. In one embodiment, a tunnel oxide layerincluding silicon dioxide is disposed on all surfaces of a siliconwafer. A polycrystalline layer is disposed on the tunnel oxide layer,the polycrystalline layer disposed on all portions of the tunnel oxidelayer except on a back side of the silicon wafer which has a ringpattern of the polycrystalline layer. In one embodiment, a tunnel oxidelayer including silicon dioxide is disposed on all surfaces of a siliconwafer. An amorphous layer is disposed on the tunnel oxide layer, theamorphous layer disposed on all portions of the tunnel oxide layerexcept on a back side of the silicon wafer which has a ring pattern ofthe amorphous layer.

In accordance with an embodiment of the present invention, in order tofabricate a passivated emitter of a solar cell, a thin tunnel oxide andheavily doped poly-silicon, both n-type and p-type, are used. Althoughsuch films may conventionally be formed, individually, in furnaces, thecombination has not been applied to fabrication of a solar cell, and themanufacturing cost to do may be prohibitive for the solar cell market.Instead, in an embodiment, the oxidation and subsequent silicondeposition are combined into a single process operation. In anembodiment, this approach can also be used to double the throughput byloading two wafers per slot in a furnace boat. In an embodiment, thesilicon is first deposited as an undoped and amorphous layer. In thatembodiment, the silicon is doped and crystallized in a later processingoperation to provide a poly-silicon layer. In an alternative embodiment,the silicon layer is formed as a poly-silicon layer in the singleprocess operation.

Embodiments of the present invention may address conventionalfabrication issues such as, but not limited to, (1) control of oxidethickness, and oxide quality, (2) contamination between oxidation andpoly deposition, (3) excessive preventative maintenance requirements,(4) throughput, or (5) control of n-poly and p-poly sheet resistance. Inaccordance with an embodiment of the present invention, several featuresfor a method of solar cell manufacturing are combined, namely thecombining of the oxidation and poly (as amorphous silicon first)deposition in a single process. In one embodiment, silicon carbide (SiC)parts are used in the furnace to extend maintenance intervals. In oneembodiment, two wafers are loaded per slot to increase throughput. Theabove embodiments may all contribute to the feasibility of manufacturingsolar cells.

In an embodiment, depositing the silicon as an amorphous layer and thendoping and crystallizing the layer in a later operation makes theprocess more controllable and improves the passivation. In anembodiment, throughput is improved by loading two wafers per slot in afurnace handling boat. In an embodiment, an SiC boat is used fordimensional stability. In an embodiment, control of sheet resistance isachieved by, instead of in-situ doped poly-silicon, depositing undopedamorphous silicon. The n and p regions are then formed selectively andcrystallized at a later, higher, temperature operation. In anembodiment, by following one or more of the approaches described herein,grain size may be maximized, sheet resistance may be minimized, andcounter-doping may be avoided.

It is to be understood that a furnace for film fabrication is notlimited to a conventional furnace. In an embodiment, the furnace is achamber for wafer processing such as, but not limited to, a verticalfurnace chamber, a horizontal furnace chamber, or a plasma chamber. Itis also to be understood that reference to an amorphous film or layerherein is not limited to an amorphous silicon film or layer. In anembodiment, the amorphous film or layer is a film or layer such as, butnot limited to, an amorphous silicon-germanium film or layer or anamorphous carbon-doped silicon film or layer.

A solar cell may be fabricated to include an emitter region. Forexample, FIG. 1 illustrates a flowchart 100 representing operations in amethod of fabricating an emitter region of a solar cell, in accordancewith an embodiment of the present invention. FIGS. 2A-2E illustratecross-sectional views of various stages in the fabrication of a solarcell including an emitter region, corresponding to operations offlowchart 100, in accordance with an embodiment of the presentinvention.

Referring to FIG. 2A, a substrate 202 for solar cell manufacturing isprovided. In accordance with an embodiment of the present invention,substrate 202 is composed of a bulk silicon substrate. In oneembodiment, the bulk silicon substrate is doped with N-type dopants. Inan embodiment, substrate 202 has a textured surface, although notdepicted in FIG. 2A.

Referring to operation 102 of flowchart 100, and corresponding FIG. 2B,a method of fabricating an emitter region of a solar cell includesforming, in a furnace, a tunnel oxide layer 204 on a surface ofsubstrate 202. In accordance with an embodiment of the presentinvention, forming tunnel oxide layer 204 includes heating substrate 202in the furnace at a temperature of approximately 900 degrees Celsius. Ina specific embodiment, heating substrate 202 in the furnace at thetemperature of approximately 900 degrees Celsius further includesheating at a pressure of approximately 500 mTorr for approximately 3minutes in an atmosphere of oxygen to provide tunnel oxide layer 204having a thickness of approximately 1.5 nanometers. In accordance withanother embodiment of the present invention, forming tunnel oxide layer204 includes heating substrate 202 in the furnace at a temperature lessthan 600 degrees Celsius. In a specific embodiment, heating substrate202 in the furnace at the temperature of less than 600 degrees Celsiusfurther includes heating at a temperature of approximately 565 degreesCelsius, at a pressure of approximately 300 Torr, for approximately 60minutes in an atmosphere of oxygen to provide tunnel oxide layer 204having a thickness of approximately 1.5 nanometers. In an alternativeembodiment, the atmosphere include N₂O.

Referring to operation 104 of flowchart 100, and corresponding FIG. 2C,the method of fabricating an emitter region of a solar cell furtherincludes, without removing substrate 202 from the furnace, forming anamorphous layer 206 on tunnel oxide layer 204. In accordance with anembodiment of the present invention, forming amorphous layer 206includes depositing amorphous layer 206 in the furnace at a temperatureless than 575 degrees Celsius. In a specific embodiment, depositingamorphous layer 206 in the furnace at the temperature less than 575degrees Celsius further includes heating at a temperature ofapproximately 565 degrees Celsius, at a pressure of approximately 350mTorr, and in an atmosphere of silane (SiH₄) to provide amorphous layer206 having a thickness approximately in the range of 200-300 nanometers.

Referring to operation 106 of flowchart 100, and corresponding FIG. 2D,the method of fabricating an emitter region of a solar cell furtherincludes doping amorphous layer 206 with dopants 208 to provide a dopedamorphous layer 210 having a first region (left side of p-n junction212) including N-type dopants and a second region (right side of p-njunction 212) including P-type dopants. In one embodiment, the dopantsare introduced from a solid-state source. In another embodiment, thedopants are introduced as implanted atoms or ions.

Referring to operation 108 of flowchart 100, and corresponding FIG. 2E,the method of fabricating an emitter region of a solar cell furtherincludes, subsequently, heating doped amorphous layer 210 to provide apolycrystalline layer 214 having an N-type-doped region 218 and aP-type-doped region 216. In accordance with an embodiment of the presentinvention, substrate 202 is composed of silicon, tunnel oxide layer 204is composed of silicon dioxide, amorphous layer 206 is composed ofsilicon, the N-type dopants are phosphorous dopants, and the P-typedopants are boron dopants. In an embodiment, both tunnel oxide layer 204and amorphous layer 206 are formed at a temperature of approximately 565degrees Celsius, and heating doped amorphous layer 210 to providepolycrystalline layer 214 includes heating at a temperature ofapproximately 980 degrees Celsius.

In order to further or complete fabrication of a solar cell, the methodabove may further include forming a metal contact above polycrystallinelayer 214. In an embodiment, a completed solar cell is a back-contactsolar cell. In that embodiment, N-type-doped region 218 and P-type-dopedregion 216 are active regions. Conductive contacts may be coupled to theactive regions and separated from one another by isolation regions,which may be composed of a dielectric material. In an embodiment, thesolar cell is a back-contact solar cell and further includes ananti-reflective coating layer disposed on a light-receiving surface,such as on a random textured surface of the solar cell.

In another aspect of the present invention, unique approaches to forminglayers on a substrate of a solar cell are provided. For example, FIG. 3illustrates a flowchart 300 representing operations in a method offorming layers on a substrate of a solar cell, in accordance with anembodiment of the present invention. FIGS. 4A-4C illustratecross-sectional views of various stages in the fabrication of solarcells, corresponding to operations of flowchart 300, in accordance withan embodiment of the present invention.

Referring to operation 302 of flowchart 300, and corresponding FIGS. 4Aand 4B, a method of forming layers on a substrate of a solar cellincludes loading, into a furnace, a wafer carrier 402 with a pluralityof wafers 404, wafer carrier 402 having one or more wafer receivingslots loaded with two wafers positioned back-to-back, such as wafers 406and 408. In accordance with an embodiment of the present invention, 50wafers are loaded into 25 slots of a carrier 402.

Referring to operation 304 of flowchart 300, and corresponding FIG. 4C,the method of forming layers on a substrate of a solar cell furtherincludes forming, in the furnace, a tunnel oxide layer 410 on allsurfaces of each of the plurality of wafers 404, e.g., on all surfacesof wafers 406 and 408, as depicted in FIG. 4C. In accordance with anembodiment of the present invention, forming tunnel oxide layer 410includes heating each of the plurality of wafers 404 in the furnace at atemperature of approximately 900 degrees Celsius. In a specificembodiment, heating each of the plurality of wafers 404 in the furnaceat the temperature of approximately 900 degrees Celsius further includesheating at a pressure of approximately 500 mTorr for approximately 3minutes in an atmosphere of oxygen to provide tunnel oxide layer 410having a thickness of approximately 1.5 nanometers. In accordance withanother embodiment of the present invention, forming tunnel oxide layer410 includes heating each of the plurality of wafers 404 in the furnaceat a temperature less than 600 degrees Celsius. In a specificembodiment, heating each of the plurality of wafers 404 in the furnaceat the temperature of less than 600 degrees Celsius further includesheating at a temperature of approximately 565 degrees Celsius, at apressure of approximately 300 Torr, for approximately 60 minutes in anatmosphere of oxygen to provide tunnel oxide layer 410 having athickness of approximately 1.5 nanometers. In an alternative embodiment,the atmosphere include N₂O.

Referring to operation 304 of flowchart 300, and corresponding FIG. 4C,the method of forming layers on a substrate of a solar cell furtherincludes, without removing the plurality of wafers 404 from the furnace,forming an amorphous layer 412 on tunnel oxide layer 410, amorphouslayer 412 formed on all portions of tunnel oxide layer 410 except on theportions in contact between wafers positioned back-to-back, e.g., asdepicted with reference to wafers 406 and 408 in FIG. 4C. In accordancewith an embodiment of the present invention, for the wafers positionedback-to-back, a ring pattern of the amorphous layer is formed on theback of each wafer, as described in more detail below with respect toFIG. 5. In an embodiment, each of the plurality of wafers 404 iscomposed of silicon, tunnel oxide layer 410 is composed of silicondioxide, and amorphous layer 412 is composed of silicon. In anembodiment, forming amorphous layer 412 includes depositing amorphouslayer 412 in the furnace at a temperature less than 575 degrees Celsius.In a specific embodiment, depositing amorphous layer 412 in the furnaceat the temperature less than 575 degrees Celsius further includesheating at a temperature of approximately 565 degrees Celsius at apressure of approximately 350 mTorr in an atmosphere of silane (SiH₄) toprovide amorphous layer 412 having a thickness approximately in therange of 200-300 nanometers. In an embodiment, the temperature is keptbelow 575 degrees Celsius to avoid crystallization of the formed layer,but not substantially below 575 degrees Celsius for the sake ofmaintaining a deposition rate suitable for high volume manufacturing.

In accordance with an embodiment of the present invention, the method offorming layers on a substrate of a solar cell further includes,subsequent to forming amorphous layer 412, applying a cleaning solutionto the back of each wafer, the cleaning solution including an oxidizingagent. A texturizing solution is then applied to the back of each wafer,the texturizing solution including a hydroxide. In one embodiment, theoxidizing agent is a species such as, but not limited to, ozone orhydrogen peroxide (H₂O₂), and the hydroxide is a species such as, butnot limited to, potassium hydroxide (KOH) or sodium hydroxide (NaOH).

The texturizing solution may provide a randomly textured (rantex)surface on a light-receiving portion of a fabricated solar cell. Inaccordance with an embodiment of the present invention, by introducing acleaning solution having an oxidizing agent prior to introducing thetexturizing solution, the texturing of the solar cell is uniform despitethe initial presence of a ring portion of a layer fabricated on thesolar cell substrate, as described below in association with FIG. 5.

A ring feature, as mentioned with respect to FIG. 4C, may be retained ona substrate of a solar cell, or may be subsequently removed.Nonetheless, a solar cell structure may ultimately retain, or at leasttemporarily include, such a ring feature. For example, FIG. 5illustrates both a cross-sectional view and a top-down view of asubstrate of a solar cell, the substrate having layers formed thereon,in accordance with an embodiment of the present invention.

Referring to FIG. 5, in accordance with an embodiment of the presentinvention, a substrate of a solar cell includes a tunnel oxide layer 504disposed on all surfaces of a wafer 502. A polycrystalline layer 506 isdisposed on tunnel oxide layer 504, polycrystalline layer 506 disposedon all portions of tunnel oxide layer 504 except on a back side of wafer502 which includes a ring pattern of polycrystalline layer 506. Inaccordance with an embodiment of the present invention, the ring patternresults from the back-to-back handling of pairs of wafers, as describedin association with FIG. 4C. In an embodiment, tunnel oxide layer 504 iscomposed of silicon dioxide, wafer 502 is composed of silicon, andpolycrystalline layer 506 is composed of silicon.

Referring again to FIG. 5, in accordance with another embodiment of thepresent invention, a substrate of a solar cell includes a tunnel oxidelayer 504 disposed on all surfaces of a wafer 502. An amorphous layer506 is disposed on tunnel oxide layer 504, amorphous layer 506 disposedon all portions of tunnel oxide layer 504 except on a back side of wafer502 which includes a ring pattern of amorphous layer 506. In accordancewith an embodiment of the present invention, the ring pattern resultsfrom the back-to-back handling of pairs of wafers, as described inassociation with FIG. 4C. In an embodiment, tunnel oxide layer 504 iscomposed of silicon dioxide, wafer 502 is composed of silicon, andamorphous layer 506 is composed of silicon.

Thus, methods of fabricating emitter regions for solar cells have beendisclosed. In accordance with an embodiment of the present invention, amethod of fabricating an emitter region of a solar cell includesforming, in a furnace, a tunnel oxide layer on a surface of a substrate.The method also includes, without removing the substrate from thefurnace, forming an amorphous layer on the tunnel oxide layer. Themethod also includes doping the amorphous layer to provide a firstregion having N-type dopants and a second region having P-type dopants.Subsequently, the amorphous layer is heated to provide a polycrystallinelayer having an N-type-doped region and a P-type-doped region. In oneembodiment, the substrate is composed of silicon, the tunnel oxide layeris composed of silicon dioxide, the amorphous layer is composed ofsilicon, the N-type dopants are phosphorous, and the P-type dopants areboron. In one embodiment, both the tunnel oxide layer and the amorphouslayer are formed at a temperature of approximately 565 degrees Celsius,and heating the amorphous layer to provide the polycrystalline layerincludes heating at a temperature of approximately 980 degrees Celsius.

1. A method of fabricating an emitter region of a solar cell, the methodcomprising: forming, in a furnace, a tunnel oxide layer on a surface ofa substrate; and, without removing the substrate from the furnace,forming an amorphous layer on the tunnel oxide layer; doping theamorphous layer to provide a first region comprising N-type dopants anda second region comprising P-type dopants; and, subsequently, heatingthe amorphous layer to provide a polycrystalline layer comprising anN-type-doped region and a P-type-doped region.
 2. The method of claim 1,wherein the substrate comprises silicon, the tunnel oxide layercomprises silicon dioxide, the amorphous layer comprises silicon, theN-type dopants comprise phosphorous, and the P-type dopants compriseboron.
 3. The method of claim 1, wherein forming the tunnel oxide layercomprises heating the substrate in the furnace at a temperature ofapproximately 900 degrees Celsius.
 4. The method of claim 3, whereinheating the substrate in the furnace at the temperature of approximately900 degrees Celsius further comprises heating at a pressure ofapproximately 500 mTorr for approximately 3 minutes in an atmosphere ofoxygen to provide the tunnel oxide layer having a thickness ofapproximately 1.5 nanometers.
 5. The method of claim 1, wherein formingthe tunnel oxide layer comprises heating the substrate in the furnace ata temperature less than 600 degrees Celsius.
 6. The method of claim 5,wherein heating the substrate in the furnace at the temperature of lessthan 600 degrees Celsius further comprises heating at a temperature ofapproximately 565 degrees Celsius, at a pressure of approximately 300Torr, for approximately 60 minutes in an atmosphere of oxygen to providethe tunnel oxide layer having a thickness of approximately 1.5nanometers.
 7. The method of claim 1, wherein forming the amorphouslayer comprises depositing the amorphous layer in the furnace at atemperature less than 575 degrees Celsius.
 8. The method of claim 7,wherein depositing the amorphous layer in the furnace at the temperatureless than 575 degrees Celsius further comprises heating at a temperatureof approximately 565 degrees Celsius at a pressure of approximately 350mTorr in an atmosphere of silane (SiH₄) to provide the amorphous layerhaving a thickness approximately in the range of 200-300 nanometers. 9.The method of claim 1, wherein both the tunnel oxide layer and theamorphous layer are formed at a temperature of approximately 565 degreesCelsius, and wherein heating the amorphous layer to provide thepolycrystalline layer comprises heating at a temperature ofapproximately 980 degrees Celsius.
 10. A method of forming layers on asubstrate of a solar cell, the method comprising: loading, into afurnace, a wafer carrier with a plurality of wafers, the wafer carrierhaving one or more wafer receiving slots loaded with two waferspositioned back-to-back; forming, in the furnace, a tunnel oxide layeron all surfaces of each of the plurality of wafers; and, withoutremoving the plurality of wafers from the furnace, forming an amorphouslayer on the tunnel oxide layer, the amorphous layer formed on allportions of the tunnel oxide layer except on the portions in contactbetween wafers positioned back-to-back.
 11. The method of claim 10,wherein for the wafers positioned back-to-back, a ring pattern of theamorphous layer is formed on the back of each wafer.
 12. The method ofclaim 11, further comprising: subsequent to forming the amorphous layer,applying a cleaning solution to the back of each wafer, the cleaningsolution comprising an oxidizing agent; and, subsequently, applying atexturizing solution to the back of each wafer, the texturizing solutioncomprising a hydroxide.
 13. The method of claim 12, where in theoxidizing agent is selected from the group consisting of ozone andhydrogen peroxide (H₂O₂), and wherein the hydroxide is selected from thegroup consisting of potassium hydroxide (KOH) and sodium hydroxide(NaOH).
 14. The method of claim 10, wherein each of the plurality ofwafers comprises silicon, the tunnel oxide layer comprises silicondioxide, and the amorphous layer comprises silicon.
 15. The method ofclaim 10, wherein forming the tunnel oxide layer comprises heating eachof the plurality of wafers in the furnace at a temperature ofapproximately 900 degrees Celsius.
 16. The method of claim 15, whereinheating each of the plurality of wafers in the furnace at thetemperature of approximately 900 degrees Celsius further comprisesheating at a pressure of approximately 500 mTorr for approximately 3minutes in an atmosphere of oxygen to provide the tunnel oxide layerhaving a thickness of approximately 1.5 nanometers.
 17. The method ofclaim 10, wherein forming the tunnel oxide layer comprises heating eachof the plurality of wafers in the furnace at a temperature less than 600degrees Celsius.
 18. The method of claim 17, wherein heating each of theplurality of wafers in the furnace at the temperature of less than 600degrees Celsius further comprises heating at a temperature ofapproximately 565 degrees Celsius, at a pressure of approximately 300Torr, for approximately 60 minutes in an atmosphere of oxygen to providethe tunnel oxide layer having a thickness of approximately 1.5nanometers.
 19. The method of claim 10, wherein forming the amorphouslayer comprises depositing the amorphous layer in the furnace at atemperature less than 575 degrees Celsius.
 20. The method of claim 19,wherein depositing the amorphous layer in the furnace at the temperatureless than 575 degrees Celsius further comprises heating at a temperatureof approximately 565 degrees Celsius at a pressure of approximately 350mTorr in an atmosphere of silane (SiH₄) to provide the amorphous layerhaving a thickness approximately in the range of 200-300 nanometers. 21.A substrate of a solar cell, comprising: a tunnel oxide layer comprisingsilicon dioxide disposed on all surfaces of a silicon wafer; and apolycrystalline layer disposed on the tunnel oxide layer, thepolycrystalline layer disposed on all portions of the tunnel oxide layerexcept on a back side of the silicon wafer which comprises a ringpattern of the polycrystalline layer.
 22. A substrate of a solar cell,comprising: a tunnel oxide layer comprising silicon dioxide disposed onall surfaces of a silicon wafer; and an amorphous layer disposed on thetunnel oxide layer, the amorphous layer disposed on all portions of thetunnel oxide layer except on a back side of the silicon wafer whichcomprises a ring pattern of the amorphous layer.